Recently, the applicant has proposed a multilayer circuit board in which layers are electrically connected using a conductive paste (in Japanese Patent No. 2,601,128). FIG. 8 shows a method of manufacturing the multilayer circuit board. As shown in FIG. 8(a), release films 501 made of polyester or the like are laminated on both surfaces of a porous substrate 502 obtained by impregnating aromatic polyamide fibers with a thermosetting epoxy resin. As shown in FIG. 8(b), through holes 503 are formed at predetermined positions in the porous substrate 502 by a laser processing method. Then, the through holes 503 are filled with a conductive paste 504 as shown in FIG. 8(c). As a filling method, the porous substrate 502 with the through holes 503 is placed on a table of a screen printing machine and the conductive paste 504 is printed directly from the top of one of the release films 501. In this case, the release film 501 at the printed side serves as a print mask and to prevent the surface of the porous substrate 502 from being polluted. Then, the release films 501 are peeled off from both the surfaces of the porous substrate 502. As a next step, metal foils 505 such as copper foils are laminated on both the surfaces of the porous substrate 502. In this state, it is heated and pressurized, thus bonding the porous substrate 502 and the metal foils 505 as shown in FIG. 8(d). In this process, the porous substrate 502 is compressed to be made thin. Simultaneously, the conductive paste 504 within the through holes 503 also is compressed, and a binder constituent contained in the conductive paste 504 is forced out, thus strengthening the adhesion between conductive constituents and between the conductive constituents and the metal foils 505. As a result, the conductive material contained in the conductive paste 504 becomes dense, and thus layers are electrically connected to each other. After that, a thermosetting resin that is a constituent of the porous substrate 502 and the conductive paste 504 are cured. Finally, as shown in FIG. 8(e), the metal foils 505 are selectively etched in a predetermined pattern, thus completing a double-faced circuit board.
However, in the aforementioned configuration and manufacturing method, when the through holes 503 are formed to be minute, the initial connection resistance increases and greatly varies. Further, the connection resistance varies depending on reliability tests such as a temperature cycling test or a pressure cooker test, which has been a problem. The problem is caused by the aspect ratio, which is the ratio of the diameter of the through holes 503 and the thickness of the porous substrate 502, approaches 1 when the through holes 503 are formed to be minute, and therefore the compressibility required for stabilizing the electric connection cannot be obtained.
In the process of peeling off the release films 501, when the through holes are decreased in diameter, the influence of the release film cannot be ignored at the ends of the through holes. In peeling off the release films, the conductive paste 504 is removed by the release films, thus preventing the through holes from being filled with the conductive paste, which has been another problem.